Two Types of Bad Blocks

There are two types of bad blocks in NAND flash memory: Early Bad Blocks and Later Bad Blocks. Early Bad Blocks are those created during the manufacturing process, and generally account for approximately 1% of capacity. These bad blocks are an unavoidable result during the production process. On the other hand, Later Bad Blocks are created in the lifetime of a flash memory over time as program and erase cycles repeatedly take place during normal operation. Roughly 7% of capacity is set aside as reserved blocks used to substitute for later bad blocks.

How Does Bad Block Management Work?

When a NAND Flash device is first used, a Bad Block Table is created, with a built-in management program in the controller checking every block. For Early Bad Blocks, where a bad block is found, the program marks it and records said block in the Bad Block Table to prevent data from being written to it again. For Later Bad Blocks, whenever the controller discovers one, it adds said block to the Bad Block Table and transfer data originally written on it to a valid block, preventing data loss. See figure below.

As shown in the figure, the controller first performs ECC (Error Correction Code) algorithms on blocks it cannot write to. Where this effort fails, the valid data is then removed to a block that is pre-reserved. After data has been removed, the bad block is marked and recorded as such in the Bad Block Table to prevent data from being written to this block again.

Conclusion

Since good blocks gradually wear out with repeated program/erase cycles, it is essential to continuously check for unusable blocks and substitute them with good ones. Modern Flash devices equipped with Bad Block Management enjoy an extended lifespan, increased reliability, and a lower read/write error rate. This has become an indispensible tool for Flash products.

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